# Speed of light

The speed of light in a vacuum (postulated to be constant for all inertial observers by the Special Theory of Relativity) is 299,792,458 meters per second (approximately 186,282.3 miles per second). As the speed of light is now used to define the SI meter, this is now the value by definition.

In physics, it is often represented in equations by the letter c, as in

λ = c / f

(wavelength of an electromagnetic wave in vacuum = the speed of light divided by the wave's frequency).

The speed of light is about one foot per nanosecond. The late computer pioneer Admiral Grace Hopper was fond of keeping foot-long lengths of wire in her purse; she used them as props for her talks, referring to them as "nanoseconds," and using them to explain how the speed of light set limitations on computing systems: no signal could possibly propagate in any wire faster than the speed of light.[1]

The speed of light is slower in any medium which is not a vacuum, and varies from medium to medium. This variation gives rise to (as a result of quantum mechanics, particularly the concept of a path of least action) the phenomenon of refraction. When a charged particle exceeds the speed of light in the medium in which it is travelling, it emits Cherenkov Radiation.

Since the speed of light in a vacuum is observed to be constant, it can be used to define distances as well. The distance that light travels in one year is known as a light-year, which is about 6 million million (6x1012) miles.

The speed of light raises questions regarding the age of the universe, which are usually summed up under the term "starlight problem".

## History

In 1676[note 1], the Danish astronomer Ole Rømer became the first person who attempted to quantitatively estimate the speed of light. His method was based on observation of the orbit of the satellites of Jupiter such as Io and evaluating their eclipse data as seen from the Earth. The Dutch scientist Christiaan Huygens was first who applied the arithmetic onto Rømer’s estimate for the maximum time delay between periodical observations of Io's eclipse by Jupiter at the Earth's nearest and farthest position with respect to Jupiter.[3] In 1849, the French physicist Fizeau developed method which enabled to measure the speed of light between two points at the Earth by means of using the light source, rotating strobe disc (spinning toothed wheel), and the mirror. He shone a light ray between the teeth of a rapidly rotating toothed wheel. A mirror reflected the beam back through the same gap between the teeth of the wheel over the round-trip distance of 17km. By varying the speed of the wheel, it was possible to determine at what speed the wheel was spinning too fast for the light to pass through the gap between the teeth, to the remote mirror, and then back through the same gap. The experiment shown that the light traveled over the known distance in 1/18000 second. The resulting speed of light through air was obtained by dividing the known distance by time. The more precise methods have been used by Americans Michelson and Newcomb.[2]

## Notes

1. Some sources state 1675.[2]

## References

1. Chiarella, Donald Joseph Gray (2002), Life in God's Management Corps, p. 14
2. 2.0 2.1 Théophile Moreux (1948). Pour comprendre la physique moderne (in French).
3. Steven Soter and Neil deGrasse Tyson:Cosmic Horizons: Astronomy At The Cutting Edge. New Press, American Museum of Natural History (2000). Retrieved on August 4, 2013.